Electrical earth borehole logging is well known, and various devices and techniques have been used for this purpose. Broadly speaking, there are two categories of devices used in electrical wellbore logging devices. In the first category galvanic electrodes are used to inject current into an earth formation, and then the current or voltage drop is measured by sensing electrodes to measure signals sensitive to formation electrical properties. In a second category inductive excitation by coils is used to induce currents within the formation, and then the magnitude and phase of the induced current is detected using also induction receiving antenna. The present invention belongs to the first category. Resistivity imaging tools have been in widespread use for several years for obtaining resistivity images of walls of boreholes drilled in an earth formation.